The present invention relates to a trace element analyzer employing a plasma, such as a plasma emission spectrometer or a plasma mass analyzer, and, more particularly, to an improved plasma analyzer for trace element analysis, capable of efficiently changing a gas supplied to a plasma and of efficiently mixing the gas in the plasma.
A representative example of a conventional plasma analyzer employing a plasma is disclosed, for example, in Japanese Patent Laid-open (Kokai) No. 64-6351. Shown in FIG. 4 are an argon (Ar) gas source 1a, a nitrogen (N.sub.2) gas source 1b, first, second and third flow meters 2a, 2b and 2c eahc provided with a valve, fourth, fifth and sixth flowmeters 3a 3b and 3c each provided with a valve, a triple-tube plasma torch 4 having an outer chamber 4a, a middle chamber 4b and an inner chamber 4c, a coil 5, a plasma 6, a sample vessel 7, a nebulizer 8, a high frequency power source 9, a sampling cone 10, a skimmer 11, vacuum pumps 12a, 12b and 12c, a mass filter 13, a detector 14 and a signal processing unit 15.
The basic operation of this conventional plasma analyzer will be described hereinafter. In an initial state, the valves of the first to third flowmeters 2a to 2c are open respectively in openings allowing corresponding gases to flow at given flow rates, respectively, and the valves of the fourth to sixth flowmeters 3a to 3c are closed. In this state, Ar gas is supplied into the outer chamber 4a and the middle chamber 4b of the plasma torch 4 through the flowmeters 2a and 2b, respectively. A sample nebulized by the nebulizer 8 is carried by Ar gas supplied as a carrier gas through the flowmeter 2c into the inner chamber 4c of the plasma torch 4. The respective flow rates of the Ar gas supplied respectively into the outer chamber 4a, the middle chamber 4b and the inner chamber 4c are regulated by the valves of the flowmeters 2a, 2b and 2c, respectively. Then, the coil 5 is energized by high frequency power supplied by the high frequency power source 9 to generate the plasma 6 by the agency of a high frequency magnetic field created by the coil 5. Ions produced in the plasma are drawn through the sampling cone 10 and the skimmer 11 into a vacuum chamber, in which the ions are analyzed by the mass filter 13 and are detected by the detector 14. A detection signal provided by the detector 14 is sent to and processed by the signal processing unit 15 to obtain mass analysis data. The mass analysis data is used for drawing a mass spectrum by a recorder.
After thus generating the plasma by using Ar gas, the valves of the fourth to sixth flowmeters 3a to 3c are opened gradually to supply N.sub.2 gas at given flow rates through the fourth to sixth flowmeters 3a to 3c into the outer chamber 4a, middle chamber 4b and inner chamber 4c of the plasma torch 4, so that Ar-N.sub.2 mixed gases of different mixing ratios are supplied respectively into the outer chamber 4a and middle chamber 4b of the plasma torch 4, and an Ar-N.sub.2 mixed gas serving as a carrier gas is supplied together with the nebulized sample nebulized by the nebulizer 8 into the inner chamber 4c of the plasma torch 4.
Then, the values of the flowmeters 2a to 2c are regulated to decrease the flow rates of the Ar gas gradually, monitoring the mass spectrum so that the peak of Ar and the peak of the objective element may not coincide with each other. If the respective peaks of Ar and the objective element coincide with each other, the valves of the flowmeters 3a to 3c are regulated so as to increase the flow rates of the N.sub.2 gas so that the respective peaks of Ar and the objective element are separated from each other. Upon the separation of the respective peaks of Ar and the objective element, the gas compositions respectively in the outer chamber 4a, the middle chamber 4b and the inner chamber 4c are sustained.
Thus, the Ar gas mixed in the plasma is changed for N.sub.2 gas to enable the measurement of the objective element without being disturbed by argon-related molecular ions.
Incidentally, it is possible that the plasma vanishes if the Ar gas is changed suddenly for N.sub.2 gas in changing the gas mixed in the plasma. Therefore, the Ar gas must be changed gradually for N.sub.2 gas to sustain the plasma. Nevertheless, the conventional plasma analyzer is not provided with satisfactory means for facilitate the gas changing operation and requires delicate operation for the adjustment of the valves of a plurality of flowmeters in combination with monitoring the mass spectrum, which requires much time and labor. Thus, the conventional plasma analyzer has problems in measuring efficiency and accessibility.